powerpc/64e: Fix amdgpu build on Book3E w/o AltiVec

There's a build failure for Book3E without AltiVec:
  Error: cc1: error: AltiVec not supported in this target
  make[6]: *** [/linux/scripts/Makefile.build:250:
  drivers/gpu/drm/amd/amdgpu/../display/dc/dml/display_mode_lib.o] Error 1

This happens because the amdgpu build is only gated by
PPC_LONG_DOUBLE_128, but that symbol can be enabled even though AltiVec
is disabled.

The only user of PPC_LONG_DOUBLE_128 is amdgpu, so just add a dependency
on AltiVec to that symbol to fix the build.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221027125626.1383092-1-mpe@ellerman.id.au